Process and circuit for reading weal magnetically recorded signals

Abstract

A process and device are disclosed for reading weak magnetically recorded signals from a photographic film provided with a transparent thin magnetic layer. The signals from a magnetic read head obtained from the moving film are pre-amplified and the pre-amplified signals are digitized, filtered, differentiated and evaluated. The output of the magnetic read head (2) is supplied to the pre-amplifier (5) in short-circuit. This means that the internal resistance R i of the magnetic head is very high compared to the input resistance R e of the amplifier connected immediately downstream of the magnetic read head coil.

I claim: 
     
       1. In a process for reading magnetically recorded signals from a photographic film provided with a transparent thin magnetic layer, the signals from a magnetic read head being pre-amplified and the pre-amplified signals being evaluated, the improvement wherein said magnetic read head is operated through a pre-amplifier in short-circuit, and wherein the pre-amplified signals are digitized, filtered, differentiated and evaluated. 
     
     
       2. A process according to claim 1, wherein an operational amplifier is used as the pre-amplifier, said operational amplifier having a very high no-load amplification A 0  of more than 10 6  and low intrinsic noise. 
     
     
       3. A process according to claim 2, wherein said operational amplifier is connected over a resistance network for positive feedback and partial compensation of the internal resistance of said magnetic read head. 
     
     
       4. In a circuit for reading magnetically recorded signals from a photographic film provided with a transparent thin magnetic layer, the signals from a magnetic read head being pre-amplified in a pre-amplifier and the pre-amplified signals being evaluated, the improvement wherein the input resistance (Re) of the pre-amplifier is very low in comparison to the internal resistance (Ri) of the magnetic read head and wherein means are provided for digitalizing, filtering, differentiating, and evaluation of the pre-amplified signals. 
     
     
       5. A circuit according to claim 4, wherein an operational amplifier is used as the pre-amplifier, said operational amplifier providing a no-load amplification A 0  of approximately 5×10 6 . 
     
     
       6. A circuit according to claim 5, wherein the operational amplifier is connected to its positive input for partial compensation of the magnetic read head internal resistance R i . 
     
     
       7. A circuit according to claim 6, wherein, in the resistance network, said positive feedback amounts to ##EQU2## which means that the compensation is approximately 80%, wherein: Rf- is the feedback resistance at the negative input terminal of the operational amplifier, Ri is the internal resistance of the magnetic read head,   Rf+ is the feedback resistance at the positive input of the operational amplifier, and   R is the resistance between ground and the positive input of the operational amplifier.   
     
     
       8. A circuit according to claim 4, wherein the output terminal of said operational amplifier is connected to an analog-digital converter of a filtering and differentiating means and a signal evaluating device.